The radiation used in optical communications is not necessarily in the visible region, and the words "optical" and "light" when used in this specification are not to be interpreted as implying any such limitation. Indeed, if silica optical fibres are used as the transmission medium, infra-red radiation is of especial usefulness because the loss minima occur in such fibres at 1.3 .mu.m and 1.55 .mu.m approximately.
Semiconductor laser structures include a p-n junction across which current flows (the conventional current from p to n) and an "active layer" in which electrons and holes combine with the production of photons by stimulated emission. The active layer has to relate suitably in band gap and refractive index to the other semiconductor regions of the structure in order to achieve a suitable degree of "confinement" of these processes to the active layer. The layers of material to either side of the active layer and in contact with the opposite faces of the active layer are known as "confinement layers".
A major field of application of semiconductor optical devices is in optical fibre communications systems. Silica optical fibres are produced in recent years have loss minima at 1.3 .mu.m and 1.55 .mu.m approximately, the latter minimum being the deeper. Accordingly, there is a special need for devices operating in the range from 1.1 to 1.65 .mu.m, especially from 1.3 to 1.6 .mu.m. (These wavelengths, like all the wavelengths herein except where the context indicates otherwise, are in vacuo wavelengths). Semiconductor lasers operating in this region of the infra-red usually comprise regions of indium phosphide and of quaternary materials, indium gallium arsenide phosphides (In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y). By suitable choices of x and y it is possible to lattice-match the various regions while varying the band gaps of the materials. (Band gaps can be determined experimentally by, for example, photoluminescence). Additionally, both indium phosphide and the quaternary materials can be doped to be p- or n-type as desired.
Semiconductor lasers comprising regions of gallium aluminium arsenide and gallium arsenide are also used for communications purposes. These operate near to 0.9 .mu.m.
The photons produced by stimulated emission, when a laser is driven at a current above a threshold current, are caused by the design of the laser to oscillate in a direction along it, in the active layer, before being emitted. During each passage through the material of the active layer the number of photons is increased to a degree determined by the balance between gain and losses in the active layer. The gain shows a peaked spectrum against emission wavelength and there is clearly an advantage in working at the gain peak of the material of the active layer.
In a Fabry-Perot laser, oscillation is caused by at least partially reflecting end-faces of the laser structure, lying at either end of the active layer. In a distributed feedback (DFB) laser, oscillation is caused by corrugations which lie in the region of the active layer, extending generally perpendicular to the length of the laser structure, the corrugations reflecting radiation in each direction along the laser structure.
Structures external to the laser can also contribute to oscillation by reflecting radiation back into the laser. Such structures include external cavities and distributed Bragg reflectors (DBR). External cavities may for instance comprise a mirror placed at a preselected distance along the emission axis. Alternatively radiation may be reflected back into the laser by means of corrugations, similar to those of a DFB laser but shifted to a position outside the laser, adjacent to the emission axis. Lasers having the latter external structure are known as distributed Bragg reflector (DBR) lasers.
In some applications, particularly coherent optical communications, it is important that the emitted radiation shows a narrow linewidth. This allows coherent detection systems, such as heterodyne or homodyne detection, to be used and much greater amounts of data to be transmitted as a consequence.
Fabry-Perot lasers have been found unsuitable, having linewidths of more than 100 MHz. It is known that DFB lasers can be fabricated having narrower emission linewidths than those of unmodified Fabry-Perot lasers, and that additional structures such as external cavities can result in narrowed emission linewidths.
However emission linewidth has tended to remain an unpredictable characteristic of different laser structures.
Considerable work has been done in trying to assess the factors which control linewidth in a laser. In the paper "Theory of the Linewidth of Semiconductor Lasers" by Charles H Henry, IEEE Journal of Quantum Electronics, QE-18 (2), February 1982 pp 259-264, a theory is presented which arrives at a broadening term (1+.alpha..sup.2), .alpha. being a fundamental parameter of the laser active material sometimes known as the linewidth enhancement factor.
As well as linewidth, .alpha. has been shown to affect the degree of transient wavelength chirping in directly modulated lasers.
There has been found, however, substantial ambiguity in the magnitude of .alpha. in long wavelength lasers. Values of .alpha. ranging from 2.2 to 6.6 have been measured or inferred. Further, a systematic dependence of .alpha. on laser length has been reported but unexplained. This latter effect is described in "Measurements of the Semiconductor Laser Linewidth Broadening Factor" by Henning and Collins, Electronics Letters 1983 19 pages 927-929.
In the paper "On the Linewidth Enhancement Factor in Semiconductor Injection Lasers", by K Vahala et al, Applied Physics Letters 42 (8), Apr. 15, 1983, it is predicted that in undoped Ga As, .alpha. will decrease with increasing excitation frequency.
Work has now been done, in making the present invention, by means of which practical embodiments of lasers may be designed which exploit a relationship between emission linewidth and operating wavelength. Significant improvements in the emission linewidths of lasers useful in optical communications for instance those having emission wavelengths of 1.3 and 1.55 .mu.m, can be achieved.
It is an object of the present invention to provide semiconductor laser assemblies which have reduced emission linewidths.
According to the present invention there is provided a laser assembly which comprises a semiconductor laser structure and means for selecting the emission wavelength of the laser structure, the selected wavelength being shorter than the wavelength of maximum gain at the threshold current, .lambda.max, by an amount such that the linewidth enhancement factor .alpha. at .lambda.max, .alpha.max, and .alpha. at the wavelength of the emitted radiation, .alpha..sub.e, are related in the manner EQU .alpha..sub.e .ltoreq.0.9.alpha.max
Preferably the selected wavelength is shorter than .lambda.max by an amount such that .alpha.max and .alpha..sub.e are related in the manner EQU .alpha..sub.e .ltoreq.0.8.alpha.max
and even more preferably, such that EQU .alpha..sub.e .ltoreq.0.7.alpha.max
Advantageously the means for selecting the emission wavelength comprises a structure which in itself will encourage a narrow linewidth emission from the laser assembly, such as a distributed feedback grating or an external cavity.
More advantageously, the means for selecting the emission wavelength comprises a combination of structures which each in themselves encourage a narrow linewidth emission from the laser assembly, such as a distributed feedback grating in combination with an external cavity.
Preferably the arrangement is such that the selected emission wavelength lies in one of the ranges 1.2 to 1.35 .mu.m and 1.48 to 1.65 .mu.m. This is important where the laser assembly is to be used in generating radiation for transmission by means of silica optical fibres.